Unless you have a used one and your budget, they could be expensive.
I thought about upgrading to a 75when I saw a BEAUTIFUL 75 on youtube with similar equipment that I have already..An HOB filter no sump! The way the tank was set up made me think I can do it easy enough with upgrading my filter & light and an extra PH and some more live rock. Thinking about it now, if I go to a 90 I think I may not be able to get away with the above. May need the sump..